The Invention relates to a method for detecting the drive state while a motor vehicle starts to roll with a decoupling in the rest position wherein a rotational direction of a n output shaft of an automatic transmission is detected, and a device for carrying out the method.
From the practice are already known motor vehicles having an automatic transmission or an automated manual mechanical transmission in which is provided a decoupling in rest position of the drive train, also designated as stand-by-control (SBC), which is activated while the vehicle is stopped or almost stopped. In rest coupling, while a vehicle is parked, such decoupling of the drive train reduces a tendency for the vehicle to creep during an introduced gear which, in conventional automatic transmissions, occurs without actuating the brake as a result of the positive engagement. A reduction of the fuel consumption is increasingly desired with regard to minimizing the tendency to creep and to the emissions of exhaust gas that appear.
When a driver""s wish to start is registered during the decoupling in rest position, e.g. by releasing the brake, the decoupling in rest position is terminated and a clutch to be engaged is pressurized according to the driving position introduced for forward or reverse travel depending on a calculated synchronization point.
The detection of the rotational direction of the output shaft of the transmission proves problematic here. To detect the output rotational speed, inductive sensors are often used in transmissions which are inaccurate in the range of low numbers of revolutions and become only active starting from higher numbers of revolutions than exist when starting a vehicle. To achieve a greater accuracy, Hall sensors are used in the practice. But with both kinds of sensors, it is not possible to detect the rotational direction of the output shaft with a single sensor since, for reasons of cost, the tooth flanks of the transmitter wheel are usually designed symmetrical and thus produce the same voltage signal independently of the rotational direction. Thus, in the evaluation of the voltage signal from which the output rotational speed is determined, the rotational direction of the output shaft cannot be determined.
Providing a second sensor or designing the transmitter wheel with asymmetric tooth flanks suggests itself for detecting the starting direction of the vehicle, but both result in considerably higher costs.
Especially in case of a sharp slope position, since a vehicle occasionally can start to roll after termination of the decoupling in rest position against the desired driving direction and introduced driving position for forward travel D or reverse travel R, but for lack of detection of the rotational direction of the output shaft by an electronic transmission control, this is not detected, the clutch to be engaged by the electronic transmission control is controlled in such situations according to the introduced driving position and not according to the actual driving state, i.e. especially not according to the actual relative rotational speed of the clutch to be engaged.
The calculation of the synchronization point and the control of the clutch to be engaged, which is usually hydraulically controlled until the synchronization point along a comfort ramp and thereafter, until a pressure jump to the main pressure level with a steeper pressure ramp, consequently are not carried out correctly when the vehicle starts contrary to the desired driving direction.
For example, despite when the vehicle""s desired forward travel rolls uphill in reverse due to a very steep slope position, the synchronization point, which is mathematically determined from the product of input rotational speed by ratio, is detected as too soon for the actual conditions. The reversal of the rotational direction toward the front is, therefore, effected with a jolt clearly noticeable by the occupants of the vehicle.
Together with such impairments to comfort, a wrong calculation of the synchronization point resulting from an undesired starting direction has, as a consequence, problems of service life for the clutch to be engaged, since there is often provided an electronic signal of a reduced maximum engine torque as a mechanical transmission protection. As a rule, such engine engagement is canceled at the synchronization point in order to not impair the spontaneity of the vehicle unnecessarily long. In case of a synchronization point calculated too soon, an engine engagement also provided as transmission protection is discontinued too soon whereby a considerable thermal load can occur in case of high load requirements of the engine.
The problem, on which the invention is based, is to provide a method for detection of the rotational direction of an output shaft of an automated transmission during starting of the vehicle after termination of a decoupling in rest position which can be carried out at minimal construction cost and makes a correct control of the clutch to be engaged possible. A problem also solved by this invention is to provide a device for carrying out the method.
According to the invention said problem is solved by a method according to claim 1 and by a device according to claim 16.
The inventive method advantageously makes use of the knowledge that the qualitative and quantitative curve of an existing rotational speed signal which, in conventional manner, is permanently detected with a transmitter wheel having symmetrical tooth flanks and a reasonably priced conventional sensor, varies according to the rotational direction of the output shaft and therewith to the starting direction of the vehicle. When the vehicle starts contrary to the desired travel direction, the qualitative rotational speed curves differ from each other strongly between the desired drive state and the actual drive state.
The invention utilizes the knowledge in the manner that a time-related behavior of a determined parameter, preferably of the transmission output rotational speed, is compared with appertaining parameter sets characteristic for drive state in forward travel or reverse travel. When as a result of the qualitative and quantitative curve of the compared parameter, a specific drive state is detected. The synchronization point is calculated according to the detected drive state.
Thus, the rotational direction of the output shaft is detected with the inventive method independently of the signal generation of the transmitter wheel, which for the inventive method, can be designed at reasonable cost with symmetrically defined tooth flanks. According to the invention, since the rotational direction for starting operations in forward travel or reverse travel in automatic transmissions or in automated manual mechanical transmissions is detected only via a software logic, bearing in mind certain secondary conditions, the expense in sensors can be kept at a minimum so that only a conventional rotational speed sensor is required for detecting the rotational speed.
With the detection of the rotational direction of the output shaft and therewith of the starting direction of the vehicle, the synchronization point of the clutch to be engaged can be advantageously calculated according to the actual drive state. This means that, in case of selected forward travel direction but actual reverse rolling of the vehicle, the calculation takes place on a slope of the synchronization point taking into account the reverse rolling so that the pressure control of the clutch to be engaged can be chronologically and qualitatively carried out correctly with the greatest possible comfort and- clutch protection. This refers to a torque reduction optionally preset by a digital engine electronic system of the maximum admissible engine torque which, with the inventive method, is canceled at the actual point of synchronization. A clutch overload is thus prevented in case of an engine engagement eventually terminated too soon.